1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system for an internal combustion engine equipped with an evaporative emission control system for purging (discharging) evaporative fuel generated from a fuel tank into an intake system of the engine to thereby control the emission of evaporative fuel into the air.
2. Prior Art
Conventionally, a control system for an internal combustion engine has been proposed, e.g. by U.S. Pat. No. 4,537,172, filed Oct. 21, 1983 (hereinafter referred to as "the first prior art system"), which comprises a fuel tank, a canister for adsorbing evaporative fuel generated from the fuel tank to temporarily store it therein, a purging passage connecting between the canister and an intake system of the engine, a flowmeter arranged in the purging passage for measuring a flow rate of a purged gas (mixture of evaporative fuel and air), and a purge control valve arranged in the purging passage at a location downstream of the flowmeter for controlling the flow rate of purged gas.
According to the first prior art system, evaporative fuel generated from the fuel tank is temporarily stored in the canister, and the stored evaporative fuel is purged into the intake system of the engine as a fuel component for combustion to be burned in a combustion chamber of the engine together with fuel injected by a fuel injection valve. Further, according to this system, the flow rate of purged gas is controlled by means of the purge control valve arranged in the purging passage so as to control the air-fuel ratio of a mixture supplied to the combustion chamber to a desired value, to thereby prevent the emission of noxious components from the engine into the atmosphere.
Further, an idling intake air control device for controlling an amount of intake air to be supplied to the engine during idling has also been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 62-3147 (hereinafter referred to as "the second prior art system"), which comprises a bypass passage bypassing a throttle valve arranged in the intake passage and an auxiliary air control valve arranged in the bypass passage, for supplying a required amount of air to the engine even during deceleration toward idling, to control the rotational speed of the engine to a desired idling speed promptly and in a suitable manner.
According to the second prior art system, the auxiliary air control valve is controlled in a feedback manner such that an actual value of the rotational speed of the engine becomes equal to the desired idling speed, whereby the optimum amount of auxiliary intake air is supplied to the engine, so as to enhance convergence of the idling speed, thereby preventing a drop of the idling speed and engine stall.
Further, an internal combustion engine has already been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 61-277837 (hereinafter referred to as "the third prior art system"), which is equipped with a fast idle device for increasing the idling speed to prevent engine stall when the engine temperature is low.
According to the third prior art system, a sensor, such as a thermistor, detects an engine coolant temperature, and when the engine coolant temperature is not higher than a predetermined value (e.g. 70.degree. C.), auxiliary air is supplied to the engine to properly control the engine rotational speed for prevention of engine stall.
In the first prior art system, however, the flow rate of evaporative fuel contained in the purged gas varies as a function of the amount of fuel stored in the canister. It is difficult to estimate the amount of evaporative fuel stored in the canister, and hence it is impossible to calculate an accurate amount of purged evaporative fuel, so that even if the flow rate of purged evaporative fuel is controlled by means of the control valve, it is impossible to accurately control the flow rate of purged evaporative fuel.
Therefore, if the second prior art system or the third prior art system is incorporated in the first prior art system and a large amount of evaporative fuel is purged into the intake passage during idling of the engine, air contained in the purged gas is supplied to the engine in addition to auxiliary air from the auxiliary air control valve or the fast idling device, so that the engine rotational speed is increased to an undesired level, resulting in degraded performance of the engine during idling.
Further, as described hereinabove, in the first prior art system, the flow rate of the purged evaporative fuel cannot be accurately calculated, so that if a large amount of evaporative fuel is drawn into the combustion chamber, the air-fuel ratio of a mixture deviates from a desired value, resulting in degraded exhaust emission characteristics.